Many portable electronic devices contain battery packs that can power the device when the user is away from home and office, and cannot plug the device into an external power source such as a common electrical wall outlet. Most such devices also include an associated or internal battery charger that will recharge the battery pack when a power cord is connected to the device and plugged into an electrical outlet. Examples of such portable electronic devices are wireless telephones and laptop computers.
When the battery pack is depleted and the user plugs the device into an external power source, e.g., an electrical outlet, the user may also wish to use the device while the battery is recharging. As a result, the current from the electrical outlet must be shared to both power the device for immediate use and recharge the depleted battery pack. However, as battery capacities increase along with the power consumption demands of portable devices, it becomes increasingly difficult to supply adequate current to run the device and to rapidly recharge the battery. This difficulty is further complicated by the need to reduce the physical size of power accessories such as the power cord and transformer with which the device is plugged into an electrical outlet.
In the cellular telephone environment, the need to share electrical current between the phone and the charger has been addressed in a number of ways. For example, in one type of phone, all the current is supplied to the phone. The phone monitors the amount of current it needs and intelligently allocates some of the remaining current to recharge the battery. This allows the phone to always have the current it needs to operate.
However, having the phone allocate the current has the following disadvantages. Hardware and programming must be added to the phone so that it can monitor and allocate current. This increases the cost of the phone. Moreover, not all the available current is consistently used and the battery cannot be charged without the phone.
Another approach is to supply all the current to the battery charger and require that the phone run off the battery while the battery is being charged. This allows all the available current to be consistently utilized, does not require additional current-allocation hardware and programming in the phone itself and allows the battery charger to be designed to charge the battery even without the phone present.
On the other hand, this approach will not allow the phone to operate if the battery is extremely depleted. Moreover, the maximum charging current tolerated by the battery must meet or exceed the peak current needed by the phone. If the maximum current tolerated by the battery is less than the peak current needed by the phone, a battery charger suitable for the phone will overpower the battery, and using a charger suited to the battery will leave the phone under-powered.
Consequently, there is a need in the art for a method and apparatus of efficiently sharing a current between an electronic device and a battery charger that avoids the problems inherent in the prior art described above.